The Two Ways Humans Feel Ticklish

Before we can even begin to explain ticklishness, we need to untangle a definitional mess that has muddled the science for decades. There are actually two completely different phenomena traveling under the same name.

The first is knismesis: that light, feathery, slightly itchy sensation you get when something brushes against your skin. Think of the feeling of a loose hair on your arm or a fly on your neck. It’s uncomfortable rather than funny, it can be self-induced and its evolutionary logic is obvious: it’s your body’s early-warning system for parasites and insects. Watch a horse flick its tail at a fly, and you’re watching knismesis at work. It is ancient, widespread across mammals and neurologically unremarkable.

The second is gargalesis: the deep, social, laughter-inducing tickle that happens when someone applies rhythmic, forceful pressure to your armpits or ribs or the soles of your feet. It produces involuntary laughter, convulsive body movements and that peculiar mixture of delight and mild panic. It cannot be self-induced. It is context-dependent, mood-dependent and socially charged in ways that knismesis simply isn’t.

What We Know So Far About Ticklishness

Gargalesis is, by any reasonable measure, a far stranger and more complex phenomenon than knismesis, and it’s the one that science has almost entirely failed to explain. The good news is that neuroscience has made real, if partial, progress — mostly by studying rats.

In a pioneering 2016 study published in Science, researchers showed that when rats were tickled by humans, neurons in the deep layers of the trunk region of the somatosensory cortex fired intensely. What was more notable, however, was that the rats didn’t just tolerate the tickling; they sought it out, jumping with what the researchers called Freudensprünge (joy jumps) and returning to the experimenter’s hand for more.

Crucially, the same cortical neurons that fired during tickling also fired during play behavior, suggesting a deep neurological link between the two. And in a finding that resonates immediately with human experience, they observed that anxiety shut the whole system down: place the rat in a bright, stressful environment, and the neurons go quiet, the laughter stops and the tickle simply doesn’t work anymore.

Then, in a 2023 study from Neuron, researchers traced the circuitry even deeper, into the periaqueductal gray (PAG): a midbrain structure involved in pain, survival vocalizations and basic emotional processing.

The researchers found that blocking the PAG completely abolished ticklishness and play behavior. The lateral columns of the PAG, in particular, lit up during tickling and play, and fell silent under anxious conditions. This is an ancient region of the brain. Its involvement tells us that ticklishness is a conserved feature of the mammalian nervous system, something we share with rats, apes and likely many other species.

The other major finding from the 2023 study concerns why you can’t tickle yourself. When your brain generates a movement, it simultaneously produces what’s called an efference copy: an internal prediction of what the resulting sensation should feel like. The brain uses this prediction to suppress, or “attenuate,” the incoming sensory signal. Since self-touch is perfectly predicted, it is perfectly cancelled. Someone else’s touch is unpredictable, and so it gets through.

This mechanism, known as sensory attenuation, is well established in sensorimotor neuroscience. This is what explains how patients with hallucinations and passivity experiences — who have a disrupted sense of what counts as “self-generated” — perceive their own self-touch as nearly as ticklish as someone else’s.

Five Questions About Ticklishness That Still Don’t Have Answers

For all that progress, the honest summary of where we stand is this: we’ve identified some of the hardware, but we still don’t understand the software. In a 2025 review published in Science Advances, researchers organized the remaining mysteries into five questions. Each sounds simple, yet none has been satisfactorily resolved:

1. Why are some body parts more ticklish than others? The armpits, ribs and soles of the feet are notoriously ticklish. But no theory adequately explains why these regions and not others. We don’t even know which specific sensory receptors mediate gargalesis — it likely involves a mix of low- and high-threshold mechanoreceptors, possibly nociceptors, but the peripheral pathway remains uncharted.
2. Do we actually enjoy being tickled? This one is trickier than it sounds. In another 2025 review from Neuroscience Research, researchers placed particular emphasis on the emotional ambivalence of being tickled. They frame it this way: vigorous tickling is not pure pleasure in the same way a massage is pure pleasure. It contains a small but real element of threat, of unpredictability — and that, paradoxically, may be exactly what makes it fun. Play, after all, is rarely purely pleasant. A little tension is the whole point.
3. Why can’t we tickle ourselves? We’ve touched on the sensory attenuation mechanism, but the precise neural implementation remains unclear. Interestingly, researchers have found that introducing a robotic time delay between a self-initiated movement and the resulting touch made people rate the sensation as slightly more ticklish. The brain’s prediction was disrupted just enough to let the sensation through. We’re close to understanding this one, but we’re not there yet.
4. Why do some people simply not get ticklish? Individual differences in ticklish sensitivity are enormous, and essentially unexplained. There seems to be a genetic component, but the specific mechanisms are unknown.
5. What is tickling actually for? This is the deepest question, and the most contentious. Tickling has been proposed as a form of mock combat that trains juveniles for real physical confrontation. It has been framed as a social bonding mechanism between parents and infants — and indeed, gargalesis is the earliest known trigger for laughter, emerging within the first year of life, before language, before complex cognition, before most other social behaviors. The Neuroscience Research study extends this further, documenting a sociosexual dimension: tickling in adults can carry explicitly erotic valence, pointing to a behavioral flexibility that a simple “parasite detection” story could never account for.

The thing about ticklishness that makes both of these reviews ultimately so compelling is that it forces a confrontation with the limits of neuroscience in a domain that seemed embarrassingly simple.

We know the general hardware: the somatosensory cortex, the PAG, the cerebellum’s predictive machinery. We know the behavior is ancient and cross-species, shared by rats, all the great apes and almost certainly many other mammals. We know it is exquisitely sensitive to social context and emotional state. We know it is, somehow, one of the very first social behaviors a human being performs.

What we don’t know is why. What is the function of a sensation that makes you laugh involuntarily at someone else’s touch, that you can’t reproduce on yourself, that vanishes the moment you’re anxious, that appears to thread together play, social bonding, emotion and physical vulnerability into a single uncontrollable giggle? Even the question of whether ticklish laughter reflects genuine joy remains genuinely unclear.

Feeling ticklish is a curiosity of human anatomy. Take the Human Anatomy IQ Test to really put your knowledge of the human body to the test.